Insitu Raman Setup for Deep Ocean Investigations Applying Two 1000 m Optical Fiber Cables and a 785 nm High Power Diode Laser

A laboratory Raman-set-up is used to demonstrate the feasibility of deep ocean measurements using 1000 m long low OH optical fibres for the excitation of the substances under study and the collection of the Raman photons. As excitation laser a 785 nm broad area distributed feedback (DFB-BA) high-power diode laser with an optical power of 1.25 W and a spectral emission width below 0.6 nm suitable for Raman spectroscopy was used.After passing beam shaping and focusing optics, a 1000 m long fiber, and a pressure-resistant optode275 mW of optical power were available at the sample. The optode also collects the Raman photons and focuses the light into the second 1000 m long fibre, which transfers the light into a spectrometer. To determine the reachable signal-to-noise ratio, Raman spectra were taken from natural minerals (sulfates, carbonates, sulfur). Even for weak Raman-signal and short integration times of 0.5 s per spectrum, the signal-to-noise ratio of 60 would be suitable for the in-situ Raman spectroscopy in deep ocean applications. Even longer optical fibres would increase to integration time up to 60 s. The shown set-up offers the opportunity to used different laser sources and spectrometers on board of a ship, only the fibres and the optode had to be suitable for deep sea conditions.